1887.] PAIRED FINS OF CERATODUS. 11 



specimen first described (fig. ] ), and its relationships to the basal 

 mesomere were the less definite of the two. It was here segmented 

 into two pieces, while it was much more intimately connected with 

 the two adjacent parameres than was the case in the former 

 example. These two fins (figs. 1 and 3) stand alone, among those 

 which I have examined, with respect to the great increase in number of 

 the parameres of the postaxial lobe, and that also bears the cartilage 

 now in question. If, as Haswell suggests (15, p. 8), this duphcation 

 of rays is reversionary to a " a pre-existing condition in which the fin- 

 skeleton consisted of branching, jointed, cartilaginous elements," 

 the only conclusion which seems to me justifiable is that the appear- 

 ance of this new element amounts to that of the reappearance of 

 one which has been lost. Haswell has described an individual (15, 

 figs. 6 and 7) in which the cartilage in question appears to have 

 been present on both sides ; and it is instructive to remark that in 

 both fins the rays of the postaxial series were, as with my specimens, 

 the more numerous. I have already stated that in the fin described 

 at the outset (fig. i), the whole skeleton was, with the exception of 

 the bar in question and the basal mesomere, very slight and leaf- 

 like. This simplification of structure, so suggestive of the reversion 

 claimed by Haswell, is seen in the basal mesomere itself. That was 

 (fig. 1, m.p.) much thinner and more flattened than is usual, and it 

 bore but one processus muscularis {tb.) instead of the three described 

 by Davidoff (c/. 7, pi. 8). All the foregoing facts point to the 

 conclusion that the newly described cartilage exists only in fins whose 

 postaxial rays remain little modified. There is, therefore, good 

 reason to regard it, let its homology prove to be what it mav, as 

 atavistic. It has disappeared in the normal fin, under a confluence 

 of the parameres of its own side, and a consequent thickening of the 

 postaxial fin-lobe. 



III. The Pectoral member of Ceratodus compared with the Pelvic 

 one of the same and the Pectoral one of the Plagiostomes. 



Hasv?ell, reviewing (15, p. 5) the well-known observations and 

 hypotheses of Balfour (1), Thacher (24, 25), and others, which 

 led them to dissent from the interj^retations of Gegenbaur and 

 Huxley, says that they, together with the facts which he brings 

 forward, seem to place it beyond a doubt that the limb of Cera- 

 todus, "so far from representing a primitive and generalized 

 type, is, as, indeed, we should expect from various other points 

 in the organization of the animal, in reality highly speciahzed, and 

 to be regarded as derivable from such simple limb-skeletons as those 

 of the Selachii." In this he was anticipated by Balfour (I) whom 

 he quotes. Balfour wrote (p. 669), when criticizing Huxley's 

 position, the leading tenets of which he supported so far as the 

 identification of the chief constituents of the fin-skeleton go, " I 

 should be much mere inclined to hold that the fin of Ceratodus has 

 been derived from a fin like that of the Elasmobranchs, by a series 

 of steps similar to those which Huxley supposes to have led to the 



